1. Field of the Invention
The present invention relates to a method for processing mixtures and compounds made from rubber and various ingredients, used to obtain tires and components thereof such as tread bands and the like: the invention is especially advantageous when used in connection with the production of rubber compounds reinforced with silica.
More particularly, the compounds which may be prepared with the method according to said invention are those of the type comprising a polymeric base having an unsaturated chain that can be crosslinked with sulphur in hot conditions, added to at least one silica filler and a silica bonding agent containing at least one sulphur atom.
The polymeric base may be any polymer or mixture of polymers, non-crosslinked, of natural or synthetic type, able to assume all the chemical/physical and mechanical features of elastomers after suitable crosslinking.
2. Description of the Related Art
European patent application No. 99.830189.9, in the name of the Applicant of the present invention, discloses a method for producing the abovementioned compounds including at least a first operating step intended for mixing the various basic ingredients so as to obtain a mixture.
This step is performed in a closed discontinuous mixer, i.e. a known device which basically comprises a container internally housing a pair of rotors turning in opposite directions, so as to mix up the ingredients introduced into the container from the top thereof.
For this purpose, said device is provided with a pneumatic cylinder located in the upper part of the container and a piston movable upwards to open the container, thereby allowing the introduction of the ingredients via special loading hoppers, and downwards so as to exert a pressure on the material processed by the rotors and located above them.
A pneumatic system located on the bottom of the container allows discharging of the mixture at the end of the mixing cycle by opening a suitable outlet.
As mentioned above, the devices likes the one described above are well known in the art: they are indeed referred to as xe2x80x9cBanbury(copyright)xe2x80x9d or xe2x80x9cIntermix(copyright)xe2x80x9d, depending on whether the rotors operate tangentially relative to each other or are inter-penetrating.
Hereinbelow the material processed during the various operating steps will be indicated by the term xe2x80x9cmixturexe2x80x9d in order to distinguish it from the xe2x80x9ccompoundxe2x80x9d, which is instead the product obtained by adding the vulcanizing system to the mixture.
The method claimed in the abovementioned patent application is aimed at providing substantially constant characteristics for those compounds having identical composition, but being produced discontinuously in separate batches, namely with the new charging of basic ingredients into the mixer after it has been emptied of the previous contents.
Indeed, not infrequently it happens that in the known processing methods, the final compounds produced from successive batches have physical and mechanical properties which are also significantly different from each other, despite being produced from the same basic ingredients and using the same recipes.
For this purpose, in the prior method there are defined at least two indirect process parameters, for example the power applied to the pair of opposing rotors and the temperature of the mixture processed by them, together with at least two direct parameters such as the speed of the rotors and the pressure exerted by the piston of the discontinuous mixer.
Thereafter, in order to obtain the desired result, the method according to the aforementioned application suggests to periodically detect one or both of the indirect parameters and, if necessary, to correct their progression over time, by varying one or both of the direct parameters.
This operating method allows to achieve the object of producing in a repeatable manner, compounds with constant characteristics or in any case that fall within a predefined range of variation.
However, this method relates only to the operating steps which occur within the discontinuous mixer and does not analyse the manufacturing process of the mixtures as a whole; in other words, the method described in European patent application No. 99.830189.9 does not analyse the consequences brought by the non-compliance with the predefined limit values for the (direct and indirect) mixing process parameters, on the other steps of the mixture and compounds processing cycle (for example on additional steps involving mixing with other mixtures in order to obtain the so-called xe2x80x9cblendsxe2x80x9d, on vulcanization, etc.).
For instance, reference should be made to the case where malfunctioning of a machine or human error could result in the temperature of the mixture or the power used by the rotors at a given instant, deviating from the predefined values: what are the consequences and what steps may be taken in order to remedy this situation?
It should be noted that the answer to said questions is of considerable importance because, as will be seen, it avoids the production of large quantities of material which must then be discarded at the end of the manufacturing process, together with all the negative consequences arising therefrom.
The object of the present invention is therefore to solve this problem by providing a method for processing mixtures and rubber compounds, whereby it is possible to operate immediately on line in order to correct or limit the negative consequences arising from fluctuations of the process parameters, beyond the predefined tolerance limits.
The invention comes from Applicant""s perception that the operating steps which make up the cycle for processing of the mixtures and compounds are of varying importance for obtaining the desired features in the end product.
Consequently, in accordance with the present invention, a processing method has been developed whereby at first reference values with associated deviation tolerances for the process parameters, during one or more operating steps, are defined.
These parameters may be the energy used by the rotors, the temperature of the mixture and the duration of mixing within the mixer or, downstream of the latter, the duration of the cycle for extraction of the semi-finished product in sheet form (hereafter also referred to in short as xe2x80x9cextrusionxe2x80x9d), as well as any other parameter according to the circumstances.
Thereafter, coefficients indicating the weight (or importance) of compliance with the tolerances, in order to achieve the desired final characteristics in a given compound, are assigned.
Throughout the manufacturing process, the aforementioned process parameters are measured for each batch and respective evaluation coefficients are attributed, depending on the values measured; finally, the coefficients attributed are added together and, on the basis of the result obtained, a classification of the semi-finished products obtained with the various batches of material is determined.
This classification provides a qualitative evaluation of the semi-finished product and, on the basis thereof, it becomes possible to operate directly along the production line in order to prevent any defective mixtures or compounds from causing damage to subsequent production.
For instance it could be considered the case where a mixture to be discarded is mixed together with other defect-free mixtures, thereby adversely affecting the xe2x80x9cblendxe2x80x9d obtained: this might require to discard large quantities of the material produced.
In particular, the method according to the invention is specifically used for the production of silica-reinforced compounds which comprise the following ingredients in variable quantities, per hundred parts by weight of polymeric base (phr), between the following limits:
In accordance with this aspect of the invention, the tolerance ranges of the process parameters are predefined for each type of compound to be produced.